take especial notice of, as it is of constant
occurrence in the works of Turner and Claude, the effects, namely, of
visible sunbeams. It will be necessary for us thoroughly to understand
the circumstances under which such effects take place.

Aqueous vapor or mist, suspended in the atmosphere, becomes visible
exactly as dust does in the air of a room. In the shadows you not only
cannot see the dust itself, because unillumined, but you can see other
objects through the dust without obscurity, the air being thus actually
rendered more transparent by a deprivation of light. Where a sunbeam
enters, every particle of dust becomes visible, and a palpable
interruption to the sight, so that a transverse sunbeam is a real
obstacle to the vision, you cannot see things clearly through it.

In the same way, wherever vapor is illuminated by transverse rays, there
it becomes visible as a whiteness more or less affecting the purity of
the blue, and destroying it exactly in proportion to the degree of
illumination. But where vapor is in shade, it has very little effect on
the sky, perhaps making it a little deeper and grayer than it otherwise
would be, but not itself, unless very dense, distinguishable or felt as
mist.

Sec. 14. They are only illuminated mist, and cannot appear when the sky is
free from vapor, nor when it is without clouds.

The appearance of mist or whiteness in the blue of the sky, is thus a
circumstance which more or less accompanies sunshine, and which,
supposing the quantity of vapor constant, is greatest in the brightest
sunlight. When there are no clouds in the sky, the whiteness, as it
affects the whole sky equally, is not particularly noticeable. But when
there are clouds between us and the sun, the sun being low, those clouds
cast shadows along and through the mass of suspended vapor. Within the
space of these shadows, the vapor, as above stated, becomes transparent
and invisible, and the sky appears of a pure blue. But where the
sunbeams strike, the vapor becomes visible in the form of the beams,
occasioning those radiating shafts of light which are one of the most
valuable and constant accompaniments of a low sun. The denser the mist,
the more distinct and sharp-edged will these rays be; when the air is
very clear, they are mere vague, flushing, gradated passages of light;
when it is very thick, they are keen-edged and decisive in a high
degree.

We see then, first, that a quantity of mist di